1. Field of the Invention
This invention is directed to an a-c drive system of the load commutated inverter type for a synchronous motor. More particularly, it is directed to a load commutated inverter a-c drive using a circuit topology for twelve-pulse operation at load commutated operation and a novel scheme for pulsed d-c current operation to avoid torque pulsations at starting and low speed.
2. Background Information
A currently available six-pulse a-c drive system includes a line side converter feeding a motor side inverter through a d-c link. The inverter providing variable frequency a-c power to the motor is of the load commutated type (LCI) which utilizes the electromagnetic force (EMF) of the motor to commutate its thyristor switches. The motor required for this type of operation is a synchronous machine. At standstill, and at low rotational speeds, the motor EMF is insufficient to effect commutation of the inverter thyristors, and other means must be employed. The current system pulses the d-c link current at low speed. During the zero dwell periods of the current pulses, the inverter thyristors recover from conduction. The thyristors next in the conduction sequence are then gated on and conduct the next d-c current pulse. The d-c current pulses are shaped by the line side converter and are essentially rectangular. This type of operation causes undesirable torque pulsations in the motor at 6K times the motor frequency (K=1, 2, 3, . . . ). The pulse width is 60 degrees electrical, thus the repetition rate of the d-c pulse signal is six times f.sub.M, the desired motor frequency.
U.S. Pat. No. 4,084,220 is directed toward reducing torque pulsations at low speeds in an LCI type a-c drive system. Instead of using one line side converter and one motor side converter connected through one d-c link, the scheme disclosed in this patent utilizes two line side and two motor side converters configured into two d-c loops. The pulsed currents forced into these loops by the line side converters consist of sinusoidal segments. In the motor windings, they are combined into pure sinusoidal currents. The pulse repetition rate of the patented scheme is 3f.sub.M.
This scheme of U.S. Pat. No. 4,084,220 theoretically causes no torque pulsations at low speed. At elevated speeds, however, when the system operates with load commutation, operation reverts to the conventional 6-pulse mode, resulting in torque pulsations of 6K order (i.e., 6K times motor frequency, where K=1, 2, 3, . . . ), even though to implement the distortion free pulsed d-c current operation requires the power electronics hardware of a twelve-pulse system. Thus, desirable performance at low speeds is obtained at the expense of two sets of LCI's, but no benefit is derived from this added complexity at elevated speeds where reduced torque pulsation is often equally desirable.
A-c drives are known in which two three-phase star connected stator windings of a synchronous motor offset 30 degrees electrically with respect to one another are fed by separate inverter bridges. Twelve-pulse operation of these systems in the line commutated mode with the inverters fed by constant d-c currents reduces torque pulsations over those generated in six-pulse systems. However, like the above mentioned six-pulse system, essentially rectangular dc pulses are provided to the inverters to commutate the thyristors at standstill and low speeds. While this reduces the harmonic components in the motor MMF over those produced in the six-pulse system, undesirable low speed torque pulsations are still present.
It is therefore a primary object of the present invention to provide an a-c drive system and a method of operation thereof which reduces torque pulsations over the entire motor speed range.
It is another object of the invention to provide such a method of operation which completely eliminates torque pulsations at standstill and low speed.
It is yet another object of the invention to provide such a method in which torque pulsations and the attendant mechanical vibration and acoustical noise can be completely eliminated up to higher motor frequencies than currently possible.